1. Technical Field
The present disclosure relates to semiconductor packaging, and in particular to leadframe-based packages, to leadframes employed in such packages, and most particularly to flat, leadless type packages and leadframes, such as, e.g., QFN packages.
2. Description of the Related Art
While significant advances have been made in semiconductor packaging, including the development of a very large number of packaging types, the majority of semiconductor devices still employ leadframe based packages. This is due to a number of reasons. In particular, leadframe packages are relatively inexpensive to produce, are well known and understood, the tooling is already in hand or readily available, etc. Additionally, leadframe packages have some advantages over many other packages, including having better thermal transmission characteristics, and more being more robust. As industry demand moves toward smaller packages and higher contact density, leadframe technology continues to evolve, so that even within the general category of leadframe packages, there are many different types. However, leadframe-based packages have at least one element in common: they all have a leadframe. Leadframes are usually stamped or chemically milled from thin metal strips, which are generally formed in rolls. Typically, a leadframe includes a die paddle on which a semiconductor die is mounted, and a plurality of contact pads that are placed in electrical contact with respective bond pads of the leadframe.
Leadframes are machined from a roll, i.e., strip, of metal foil, usually copper, which can be plated to prevent oxidation that might otherwise interfere with making good electrical contact. Chemical milling is typically used to form leadframes that have very fine details, such as with near chip-scale frames and frames that employ a large number of leads.
FIG. 1 is a plan view of a portion of an exemplary leadframe strip 100 for a quad flat no-lead (QFN) semiconductor package. The strip 100 of FIG. 1 includes a plurality of die paddles 102, and a plurality of leads 104 associated with each die paddle. Support stringers 106 extend between a support frame 108 and the die paddles 102, and the bond pads are attached directly to the frame. During the packaging process a semiconductor material die is attached to the die paddle 102, usually via a thermally conductive adhesive. Bond wires are attached at one end to contact pads of the semiconductor die, and to respective ones of the leads 104 at the other end, thereby placing the leads in electrical contact with the contact pads of the semiconductor die. Prior to attaching the semiconductor dies, a carrier tape is adhered to the back side of the leadframe strip 100, and after the wire bonding step, a molding compound is applied over the semiconductor die and bonding wires to form a protective package body, the approximate dimensions of which are indicated by the dashed lines 110 of FIG. 1. The carrier tape prevents the molding compound from covering the back side of the leadframe, so that the back sides of the die paddles 102 and leads 104 are exposed for contact with a circuit board. After the molding compound has been cured, the carrier tape is removed and the packages are punched from the leadframe strip. In the punching process, portions of the leadframe that protrude from each package are trimmed very close to the face of the package. The exposed die paddle on the back side of each package permits a good thermal contact between the semiconductor die and a circuit board, while the exposed leads 104 enable mounting of the package to contacts on the circuit board that are positioned inside the footprint of the package, which reduces the surface area occupied by the package on the circuit board.
There is continual industry pressure to reduce the thickness of semiconductor packages, especially for use in consumer electronic devices such as cell phones and PDA devices. In some cases, OEM manufacturers require that a package height be no more than 400 μm, and it is likely that even thinner packages will be required in the future. One problem associated with leadframe-based packages is that there is a minimum thickness requirement for leadframe material. Typically, leadframe strips cannot be much less than about 100 μm, because thinner material becomes virtually impossible to handle without damage. It lacks the strength and rigidity to withstand normal handling and the processes to which it is subjected during typical manufacturing operations. To the extent that special tooling and procedures could be devised to successfully handle thinner material, this would represent a significant additional expense and would eliminate one of the primary advantages of leadframes over other packaging formats, i.e., the ability of many existing automated systems to perform some or all of the processing steps required for more advanced package designs.
FIG. 2 is a simple diagrammatic side view of a portion of a leadframe strip 120 according to known art. A carrier strip 122 is provided, on which separate die paddles 124 and leads 126 are formed by a metal deposition process. The carrier strip acts as a support substrate so that the die paddles and bond pads can be made much thinner than the nominal 100 μm limit of the traditional punched or chemically milled leadframe strips. Additionally, because the elements of the leadframe are individually formed, there is no requirement for a support frame, which permits greater freedom to the designer because there is no requirement that every feature have an unobstructed path to the perimeter. Furthermore, because the features are not required to be self-supporting, elements that are very thin or closely spaced can be formed, where similar elements on a traditional leadframe would bend or short against other elements. Unfortunately, the processes necessary to manufacture leadframes by metal deposition are expensive and time consuming.